The present invention relates to optical devices and especially to optical waveguide filters for integrated optics.
This specification refers to several published articles. For convenience, the articles are cited in fill in a numbered list at the end of the description and cited by number in the specification itself. The contents of these articles are incorporated herein by reference and the reader is directed to them for reference.
In the context of this patent specification, the term xe2x80x9coptical radiationxe2x80x9d embraces electromagnetic waves having wavelengths in the infrared, visible and ultraviolet ranges.
The terms xe2x80x9cfinitexe2x80x9d and xe2x80x9cinfinitexe2x80x9d as used herein are used by persons skilled in this art to distinguish between waveguides having xe2x80x9cfinitexe2x80x9d widths in which the actual width is significant to the performance of the waveguide and the physics governing its operation and so-called xe2x80x9cinfinitexe2x80x9d waveguides where the width is so great that it has no significant effect upon the performance and physics or operation.
The theory of optical filters is well known in the art. Filters often are an integral part of optical systems. With the growing demand for dense-wavelength-division-multiplexing (DWDM) systems, the need for better filters with improved spectral characteristics (low sidelobe levels, narrow bandwidths, and high reflectance) is increasing. The ideal reflection filter response is characterized by unity reflection in the reflection band, fast roll-off of the response in the transition region and zero transmission outside of the reflection band. All existing filter technologies have non-ideal characteristics. Close to unity reflection is typically achieved within the band of interest, while out-of-band regions suffer from high sidelobe levels. High sidelobe levels and non-unity reflection in the reflection bandwidth can cause inter-channel cross talk in closely spaced DWDM channels. To avoid this effect, channel spacing is increased, reducing the capacity of communication networks. To remedy this issue, methods to improve filter characteristics are currently being sought.
Many filter technologies currently exist in the art, including multilayer interference filters or mirrors, fiber Bragg gratings, metallic gratings, and corrugated-waveguide gratings. Generally, these filter-types employ Bragg reflection. None of these technologies are without limitations that impede on the ideality of the spectral characteristics of their filters. Basic filter design concepts are found throughout the art and apply to all of the aforementioned technologies. The concept of Bragg reflection is linked to the period or pitch p of the grating structure, which dictates the particular wavelength, or Bragg wavelength xcexB, to be reflected. Reflection within the structure is created by the constructive addition of micro-reflections occurring at each interface, as caused by a refractive index perturbation introduced along the length of the structure. A main difference among the existing technologies in the art is the physical construction of this refractive index perturbation.
The multi-layer filter or mirror is composed of a superposition of thin films with different refractive indices and layer thicknesses t. The proper selection of the thickness of each film arranged in a periodic, chirped or other fashion would create a strong reflection at a particular wavelength and angle of incidence.
Fiber Bragg gratings (FBG""s) are created by photo-inducing a variation or perturbation of the refractive index in the core of an optical fiber constructed from appropriate materials. Numerous fabrication methods exist; the foremost method is based on the concept of a phase mask. A phase mask is a silica plate onto which a corrugated grating structure is etched. The design of the phase mask pitch is directly related to the Bragg period of the grating that will be photo-induced into the fiber core. A large body of work exists on the design and construction of FBG""s. Despite the apparent maturity of this technology, fiber Bragg gratings are limited by fabrication issues and non-ideal out-of-band characteristics.
Corrugated-waveguide gratings are another filter technology. They have found great use in the field of distributed-feedback (DFB) lasers. A corrugated grating, with pitch p and variable profile (square, sinusoidal, or other) is etched into a semiconductor waveguide layer, patterned using photolithography techniques.
A corrugated metal grating comprises such a semiconductor grating coated by a thin metal film (several nm thick) to form a coated corrugated-waveguide grating or metal grating. The art of metallic gratings has emerged from the discovery of absorption anomalies, which are caused by the excitation of surface-plasmon modes at a single semiconductor-metal interface. The patterning of the metal layer in the form of a metal grating improves the coupling and mode selection of an external electromagnetic excitation to a surface-plasmon mode. These devices are also being investigated for their photonic band-gap structures.
Reference [1] discloses a Quantum-Cascade (QC) laser which employs a metal grating and makes use of surface-plasmon modes excited at a single xe2x80x9cinfinitely widexe2x80x9d metal-semiconductor interface. The grating is included within the semiconductor laser structure to enhance mode selection, that is, create single mode selection through Bragg reflection. The grating comprises deposited strips of titanium covered by a thick evaporated layer of gold to create a metal-grating structure with alternating stripes of pure Au and Ti/Au defining the refractive index perturbation.
At present, none of the above-described technologies are without limitations.
An object of the present invention is to provide an alternative to the above-mentioned filter design technologies, and/or to mitigate some of the limitations of the prior art.
According to one aspect of the present invention, there is provided a grating, suitable for filtering optical radiation, comprising a plurality of concatenated grating sections, physical characteristics of each section differing from physical characteristics of each adjacent section so that the propagation constants of adjacent sections differ, at least some of the sections each comprising a waveguide structure formed by a thin strip (100) of a material having a relatively high free charge carrier density surrounded by material having a relatively low free charge carrier density, the strip having finite width (W) and thickness (t) with dimensions such that optical radiation having a wavelength in a predetermined range couples to the strip and propagates along the length of the strip as a plasmon-polariton wave.
The strip may comprise for example, a metal or a highly doped semiconductor. The surrounding material may comprise, for example, an insulator or undoped or lightly doped semiconductor.
A plasmon-polariton filter or grating embodying this invention may be constructed by patterning a section of the waveguide strip, that is, varying its width w along the direction of propagation to create a physical perturbation in the waveguide over a certain section of length L. Another grating embodiment introduces a pattern of narrow metal gaps of appropriate size between metal strips over a certain section of the strip of length L to create the physical perturbation. The xe2x80x98gapsxe2x80x99 may be filled by either the surrounding material, another material, e.g. dielectric, or another strip having a different high free charge carrier density. The pattern may take any form that adheres to the constraints of the applied fabrication method.
The plurality of concatenated grating sections may comprise a series of cells, each cell comprising two grating sections, said series comprising a first set of cells (xcex91, xcex92, . . . xcex9S) and a second set of cells (xcex91xe2x80x2, xcex92xe2x80x2, . . . xcex9Sxe2x80x2), different from each other and interleaved alternately cell by cell.
Preferably, the first set is equivalent to the second set transposed longitudinally.
According to a second aspect of the invention, there is provided a method of designing a grating suitable for filtering optical radiation within a specified range of wavelengths and formed from a waveguide strip surrounded by a dielectric material, the method comprising the steps of:
(i) using a numerical analysis method, deriving for said specified wavelengths, a waveguide strip of a particular material, and a particular surrounding dielectric material, normalized phase constant (xcex2/xcex20) and normalized attenuation constant (xcex1/xcex20) for a particular waveguide strip thickness and each of several waveguide widths, or for a particular waveguide width and for each of a plurality of waveguide thicknesses;
(ii) determining a particular structure for the grating as comprising a series of strips having a predetermined overall length, adjacent strips in the series having different widths, or a series of strips all having the same width and with spaces between adjacent ones of the strips, or a series of strips having spaces between adjacent strips, alternate strips having different widths, and selecting for each of said strips a particular length;
(iii) using the normalized phase constants and normalized attenuation constants derived in step (i), obtaining the complex effective refractive index (xc3x1eff=xcex2/xcex20xe2x88x92jxcex1/xcex20) for each of said strips said series;
(iv) constructing an equivalent stack of dielectric slabs, each slab taking on the complex effective refractive index of the corresponding strip in said series of strips; and
(v) deriving the optical response of the equivalent stack,
(vi) if the derived optical response is not the desired optical response, repeating steps (ii), (iii), (iv) and (v) with different parameters for the grating; and
(vii) if the derived optical response is the desired optical response, fabricating the grating with said particular structure.
The optical response may be derived using a transfer matrix method or coupled mode theory. Where the grating is uniform, however, the optical response also may be derived using the Bloch theorem.
Thus, the spectral behavior of the plasmon-polariton gratings may be determined through a fill-wave Transfer Matrix Method algorithm that incorporates the results of a Method of Lines (MoL) modeling tool that solves for the complex refractive index of the fundamental mode supported by the finite width symmetrical metal waveguide sections. The plasmon-polariton gratings are modeled as an equivalent stack of thin dielectric slices. The group delay of the grating may be found numerically by applying a finite-difference formula of O(h4).
Numerous design architectures are supported by the plasmon-polariton gratings, among them: uniform periodic gratings; non-periodic or chirped gratings; step-chirped gratings; interleaved non-periodic or chirped gratings; interleaved periodic gratings; apodized structures and higher order structures. All of the above mentioned design architectures are disclosed in full hereafter in the detailed description of the present invention
Other advantages and features of the present invention will be readily apparent from the following detailed description, examples of preferred embodiments, the drawings and claims.